This invention relates to a deflection yoke for a cathode ray tube configured to suppress or eliminate "ringing".
The quality of a CRT display is heavily influenced by various characteristics of the yoke whose electromagnetic fields control the deflection of the electron beam(s) emitted by the electron gun(s) of the tube, and any unnecessary or spurious field components generated by the deflection yoke adversely affect the quality of the image displayed on the CRT screen.
One such quality detractant or distortion is termed ringing; it is caused by mutual interference or cross-coupling between the electromagnetic fields generated by the horizontal and vertical deflection coils of the yoke. There are two primary types or categories of ringing. One is caused by sub-harmonic vibrations or resonances in the horizontal deflection current, and produces shading stripes in the raster as shown at 301 in FIG. 1(a). The other type of ringing is caused by parasitic vibration currents induced in the vertical deflection coil by the horizontal deflection field, and results in undulations in the horizontal scan lines as shown at 302 in FIG. 1(b).
There are two general types of deflection yokes for cathode ray tubes. One type is termed a "semi-toroidal" yoke, in which the horizontal deflection coil comprises a pair of saddle windings and the vertical deflection coil is wound around a toroidal core. The other type is termed a "saddle-saddle" yoke, in which both the vertical and horizontal deflection coils comprise respective pairs of saddle windings.
Semi-toroidal yokes are particularly prone to ringing due to a relatively high degree of electromagnetic coupling between the horizontal and vertical deflection fields. One approach to minimizing such ringing in a semi-toroidal yoke is disclosed in Japanese Utility Model No. 57-33560 as illustrated in FIG. 2, wherein reference numeral 1 designates a toroidal vertical deflection coil, 2 is a high voltage coil, 3 is a low voltage coil, 4 and 5 are center taps, 6 designates a line connecting the high and low voltage coils, 7 is an input terminal of the high voltage coil, 8 is an input terminal of the low voltage coil, and 9 is a toroidal core around which the coils are wound. A capacitor 100 and a resistor 101 are connected in series between the two center taps.
FIG. 3 shows an equivalent circuit of the vertical deflection coil illustrated in FIG. 2, wherein L1, L2, L3 and L4 designate the inductances of the coil portions 2a, 2b, 3a and 3b, whose values are substantially equal. Inherent, distributed capacitances C1, C2, C3 and C4 are also associated with such coil portions, however, and these give rise to unbalanced ringing currents due to the different voltages induced in the respective coil portions. The comparatively high frequency ringing currents are thus suppressed, or more accurately balanced to minimize their effects, by providing a low impedance path between the center taps 4, 5 of the coils as represented by the capacitor 100 and resistor 101.
Saddle-saddle type yokes in which the vertical deflection coils are not directly wound around a toroidal core are generally more immune to ringing current distortions due to their reduced degree of electromagnetic coupling between the horizontal and vertical deflection coils, and have thus found increased acceptance in the industry.
In recent years, high resolution display monitors have been developed for use with CAD/CAM systems and the like, and such monitors employ high horizontal sweep frequencies. While saddle-saddle type yokes are preferred for these applications, when the horizontal sweep frequency exceeds 64 kHz and approaches 90 kHz, ringing distortions as shown in FIGS. 1(a) and 1(b) again become a problem.
FIG. 4 shows a vertical sectional view of a saddle-saddle type deflection yoke taken transverse to the center axis of the CRT, wherein reference numerals 21 and 22 designate the upper and lower horizontal deflection coils of a pair of saddle windings, 31 and 32 similarly designate the left and right vertical deflection coils of a pair of saddle windings, and 1000 designates the horizontal deflection field generated by the coils 21 and 22. The vertical deflection coils necessarily overlap and flank the horizontal deflection coils, which results in their unavoidable coupling with the horizontal deflection field 1000. If the horizontal and vertical saddle winding pairs are symmetrically configured and symmetrically positioned with respect to the CRT, any spurious signals induced in the vertical coils by the horizontal deflection field are completely cancelled. Such perfect symmetry is difficult if not impossible to achieve as a practical matter, however, and small unbalanced signals are thus induced in the vertical deflection coils. These signals produce undesired ringing distortions in the displayed image, which are enhanced by the inherent distributed capacitances of the vertical deflection coils. The phenomenon is similar to that of a transformer, with the horizontal deflection coils 21, 22 representing high frequency primary windings and the vertical deflection coils 31, 32 serving as secondary windings.